Surface-emitting device, front light, and liquid crystal device

ABSTRACT

There is provided a surface-emitting device, such as a front light, in which visibility is not deteriorated even when a point light source, such as a light-emitting diode, is used. 
     A surface-emitting device  20  forming a front light includes point light sources  21 , such as light-emitting diodes, a transmissive light guide plate  22  made of acrylic resin, polycarbonate resin, or the like by injection molding or by other methods, a transmissive light-scattering plate  23  bonded onto an end face  22   d  of the light guide plate  22  opposite from the side of an end face  22   c  where the point light sources  21  are placed, and a reflection plate  24  bonded onto the surface of the light-scattering plate  23.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface-emitting device, a frontlight, and a liquid crystal device, and more particularly, to thestructure of a surface-emitting device suitably used as a front lightfor a liquid crystal display device.

2. Description of Related Art

Conventionally, reflective liquid crystal display panels that do notconsume large amounts of power are used in portable devices and thelike, whereas the displays thereof are not visible in dark conditions,such as during the night. In contrast, since transmissive liquid crystaldisplay panels have backlights, displays thereof are visible even in dimconditions. However, the backlights consume large amounts of power, andthe displays are rather difficult to view outdoors in bright daylight.

In order to solve the above problems, a liquid crystal display devicehaving a front light serving as a surface-emitting device has beenproposed in which a light guide plate is placed in front of a reflectiveliquid crystal display panel, light from a light source, such as a coldcathode-ray tube, placed adjacent to the end of the light guide plate,is introduced into the light guide plate, and the light is emitted fromthe surface of the light guide plate toward the liquid crystal displaypanel, thereby allowing the display to be visible even in a darkenvironment. In the liquid crystal display device having a front light,since the liquid crystal display panel is visible through the lightguide plate in the daytime, it can be used as a normal reflective liquidcrystal display panel. In a dark environment, the liquid crystal displaypanel is illuminated by lighting the front light, so that the display isvisible.

The above-described conventional front light has a configuration inwhich light is substantially uniformly introduced into the light guideplate by using a linear light source, such as a cold cathode-ray tube,so as to illuminate the liquid crystal panel. In small devices, such asportable devices, it is sometimes impossible, from the viewpoint of costand capacity, to use the cold cathode-ray tube or the like, and the coldcathode-ray tube consumes too large amount of power.

For this reason, it may be possible to reduce the cost, size, and powerconsumption of the devices by using a point light source, such as alight-emitting diode, which is inexpensive and which consumes a smallamount of power. In this case, however, since the light-emitting diodeis a point light source and has directivity in the light emittingdirection, light is not uniformly emitted toward the light guide plate.As a result, the light intensity distribution (in-plane distribution) ofillumination light for the liquid crystal display panel is madenonuniform, and this impairs visibility in a dark environment.

SUMMARY OF THE INVENTION

As shown in FIG. 5, in a case in which a front light 10 is providedusing a light guide plate 12 composed of convex portions, each having atriangular cross section and consisting of a gently inclined face 12 aand a steeply inclined face 12 b, arranged on its surface in paralleland in stripes in plan view, the front light 10 is set in most cases tobe viewed from the F-direction in the figure, which is slightly inclinedwith respect to the normal to the surface of the light guide plate 12,in order to enhance effective display brightness during illumination. Asshown in FIG. 6, when a linear light source, such as a cold cathode-raytube, is used as a light source, multiple bright lines are viewed astransverse lines due to light leaking from the steeply inclined faces 12b. As shown in FIG. 7, if point light sources 13, such as light-emittingdiodes, are used as light sources in this case, short bright lines areviewed in the form of bars along the light guiding direction from thelight source, which is different from the case of the linear lightsource, such as a cold cathode-ray tube, and therefore, visibility isfurther reduced, compared with the case of the linear light source.

Accordingly, the present invention has been made to overcome the aboveproblems, and an object of the present invention is to provide asurface-emitting device, such as a front light, in which visibility isnot reduced even when using a point light source such as alight-emitting diode.

An Exemplary embodiment that the present invention provides to overcomethe above problems is a surface-emitting device including a transmissivelight guide plate for emitting from its surface light propagatingtherein in a predetermined direction along the surface, and a lightsource placed adjacent to an end of the light guide plate so as tointroduce light into the light guide plate in the other direction thatis different from the predetermined direction, wherein a reflectinglayer is disposed at an end of the light guide plate different from theend where the light source is placed, so as to reflect light, whichpropagates inside the light guide plate in the other direction, innearly the predetermined direction inside the light guide plate.

According to this exemplary embodiment, light introduced from the lightsource into the light guide plate travels inside the light guide plate,is reflected by the reflection plate, travels in nearly thepredetermined direction, and is emitted from the surface of the lightguide plate. Since this allows a long optical path length from the lightsource to the emitting position, in-plane uniformity of emitted lightcan be enhanced when the light source has directivity, when the lightsource is a point light source, or when the distribution of lightintroduced from the light source into the light guide plate isnonuniform. When the surface-emitting device is used as a front lightplaced in front of various display devices, since nonuniformity ofdistribution of leakage light leaking frontward can be reduced,visibility can be improved. The other direction described above may be,for example, a direction along the surface of the light guide plate andopposite from the predetermined direction.

According to the above exemplary embodiment, it is preferable tointerpose a light-scattering layer between the light guide plate and thereflecting layer. When the light-scattering layer is placed before thereflecting surface, light is scattered before and after reflection,in-plane uniformity of emitted light can be further enhanced.

According to the above exemplary embodiment, the light source may havedirectivity in the light emitting direction inside the light guideplate, and the light source may be a point light source. Particularly inthese cases, the present invention is effective.

According to the above exemplary embodiment, it is preferable that thelight source be a light-emitting diode. By using the light-emittingdiode, the size and weight of a device having the surface-emittingdevices built therein can be reduced, and manufacturing costs can alsobe reduced.

According to any one of the above exemplary embodiment, it is preferablethat the light guide plate be structured so as not to emit the lighttraveling in the other direction from its surface before the lightreaches the reflecting layer. When the light guide plate is formed so asnot to emit light traveling in the other direction of the lightintroduced from the light source, since all the optical path length oflight emitted from the surface of the light guide plate can be madelonger than the distance from the light source to the reflecting layer,in-plane uniformity of emitted light can be enhanced.

According to any one of the above exemplary embodiment, it is preferablethat the surface of the light guide plate be provided with a convexportion or a concave portion having an inclined face for emitting lightfrom the surface.

According to the above exemplary embodiment, it is preferable that theconvex portion or the concave portion be provided with a gently inclinedface formed to face the other direction, as viewed from the inside ofthe light guide plate, and inclined at a small angle to the surface ofthe light guide plate, and a steeply inclined face formed to face thepredetermined direction, as viewed from the inside of the light guideplate, and inclined at a large angle to the surface of the light guideplate. In this case, it is preferable that the convex portion consistsof the gently inclined face and the steeply inclined face, have atriangular cross section, and be formed in stripes in plan view. In thiscase, when the convex portion is formed on the surface opposite from thelight emitting direction and the predetermined direction and the otherdirection are opposite from each other, the steeply inclined face isformed on the light-source side of the convex portion, and the gentlyinclined face is formed on the side opposite from the light source.

According to any one of the above exemplary embodiment, it is preferablethat the light source and the light guide plate be constructed as afront light to be placed in front of the panel surface of a liquidcrystal device.

In accordance with another exemplary embodiment, there is provided afront light including a transmissive light guide plate for emitting fromits surface light propagating therein in a predetermined direction alongthe surface, and a light source placed adjacent to an end of the lightguide plate so as to introduce light into the light guide plate in theother direction, which is different from the predetermined direction,wherein the light guide plate allows sight therethrough, and areflecting layer is disposed at an end of the light guide platedifferent from the end where the light source is placed, so as toreflect light, which propagates inside the light guide plate in theother direction, in nearly the predetermined direction inside the lightguide plate.

According to this above exemplary embodiment, light introduced from thelight source into the light guide plate propagates inside the lightguide plate, is reflected by the reflection layer, propagates in theother direction, and is emitted from the surface of the light guideplate. Since this allows a long optical path length from the lightsource to the emitting position, even when directivity lies in the lightemitting direction of the light source, it can be reduced. This makes itpossible to improve in-plane uniformity of illumination light and toreduce nonuniformity of leakage light, thereby reducing deterioration ofvisibility resulting from directivity of the light source.

According to above exemplary embodiment, it is preferable to place alight-scattering layer before the reflecting surface of the reflectinglayer. According to this front light, since light scattered by thelight-scattering layer at a distance from the light source is emittedfrom the light guide plate, operations equivalent to those of the lightsource having low directivity can be obtained even when directivity liesin the light emitting direction of the light source, and visibility canbe further improved. In particular, it is possible to further reducedirectivity in a region closer to the light source, as compared with acase in which a light-scattering plate is placed on the incident surfaceof the light guide plate in the conventional front light structure. Thisreduction allows a light-scattering plate having a relatively lowscattering intensity to be used, which reduces light loss.

According to above exemplary embodiment, the light source may havedirectivity in the light emitting direction in the light guide plate.

According to above exemplary embodiment, the present invention iseffective particularly in a case in which the light source is a pointlight source, and it is preferable that the light source be alight-emitting diode. According to any one of above exemplaryembodiment, it is particularly effective when the light guide plate isconstructed so as not to emit light, traveling in the other direction,from the surface before it reaches the reflecting layer.

It may be possible to construct a reflective liquid crystal device inwhich the above front light is placed in front of a liquid crystalpanel. In this case, high visibility can be obtained even in both brightand dark environments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general schematic sectional view showing the structure of asurface-emitting device (a front light) according to an embodiment ofthe present invention.

FIG. 2 is a general structural view showing the general structure of aliquid crystal device using the embodiment.

FIG. 3 is a partially sectional general plan view showing the state oflight distribution in plan in the embodiment.

FIG. 4 is a general plan view showing the general planar structure ofthe liquid crystal device using the embodiment.

FIG. 5 is a general schematic sectional view showing the generalstructure of a conventional front light.

FIG. 6 is an outward explanatory view showing the outward view of theconventional front light in which a linear light source is used as alight source.

FIG. 7 is an outward explanatory view showing the outward view of theconventional front light in which point light sources are used as alight source.

DETAILED DESCRIPTION OF THE EMBODIMENTS

An embodiment of a surface-emitting device and a liquid crystal deviceaccording to the present invention will now be described in detail. FIG.1 is a schematic sectional view showing the general structure of asurface-emitting device 20 according to the present invention. Thisembodiment preferably consists of comprises point light sources 21, suchas light-emitting diodes, a transmissive light guide plate 22 made ofacrylic resin, polycarbonate resin, or the like by injection molding orby other methods, a transmissive light-scattering plate 23 bonded ontoan end face 22 d of the light guide plate 22 on the side opposite froman end face 22 c where the point light sources 21 are placed, and areflection plate 24 bonded onto the surface of the light-scatteringplate 23. The light-scattering plate 23 and the reflection plate 24 maybe held by another member, such as a casing, while they are simply incontact with the end face 22 d of the light guide plate 22, or may beformed on the end face 22 d by a physical or chemical method.

On the surface of the light guide plate 22, multiple convex portions,each composed of a gently inclined face 22 a and a steeply inclined face22 b, are arranged in stripes and in parallel. In FIG. 1, the number ofthe convex portions is limited to three, and the shape of the convexportions is enlarged and schematically shown for easy understanding.Even when light, which propagates along the surface of the light guideplate 22, impinges on the rear face of the light guide plate and thegently inclined faces 22 a, it is totally reflected without leakingoutside because of the high refractive index of the light guide plate,and propagates again along the surface inside the light guide platewhile hardly changing its propagating direction. Light is emitted asleakage light 22B toward the front side of the light guide plate 22 whenit is incident on the steeply inclined faces 22 b at a smaller anglethan the critical angle, and is totally reflected when it is incident onthe steeply inclined faces 22 b at a larger angle than the criticalangle. When the reflected light reaches the rear face of the light guideplate 22 and is incident on the rear face at a larger angle than thecritical angle, it is totally reflected and propagates again inside thelight guide plate. In contrast, when the reflected light is incident onthe rear face at a smaller angle than the critical angle, it is notreflected and is emitted downward as illumination light 22A from therear face of the light guide plate 22. In this case, since the averageemitting direction of the illumination light 22A becomes closer to thedirection of the normal to the surface of the light guide plate 22 asthe inclination of the steeply inclined faces 22 b increases,illumination efficiency is improved. Simultaneously, since theproportion of light, which is incident on the steeply inclined faces 22b at a smaller angle than the critical angle, increases, more leakagelight 22B is not totally reflected and is emitted from the steeplyinclined faces 22 b. Since visibility becomes lower as the amount of theleakage light 22B increases, the inclination angle of the steeplyinclined faces 22 b is appropriately set to be, for example,approximately 30° to 50° . The light emitting structure of the lightguide plate 22 for emitting light from the rear face is not limited tothe above-described convex portions.

In this embodiment, light emitted from the point light sources 21travels to the left side in the figure along the surface of the lightguide plate for a time. While a part of the light impinges on the gentlyinclined faces 22 a and the rear face, most of it is totally reflectedby the gently inclined faces 22 a and the rear face and still propagatesinside the light guide plate 22. The inclination angle of the gentlyinclined faces 22 a is determined so that light, which is emitted fromthe point light sources 21 and impinges on the gently inclined faces 22a, is not totally reflected and does not leak outside, and so that thenumber and area of the gently inclined faces 22 a are not reduced due tothe increase in formation pitch of the convex portions resulting fromthe increase in length of the oblique lines of the gently inclined faces22 a in cross section, as viewed from the light traveling direction.

When the light reaches the left end face 22 d of the light guide plate22 in the figure, it passes through the light-scattering plate 23, isreflected by the reflection plate 24, and travels to the right in thefigure inside the light guide plate 22. Since the steeply inclined faces22 b are formed on the front side in the traveling direction of thereflected light, most of the light, which impinges on the steeplyinclined faces 22 b, is totally reflected and is emitted as illuminationlight 22A from the rear face of the light guide plate 22. A part of thelight passes as leakage light 22B through the steeply inclined faces 22b.

FIG. 2 shows the general configuration of a liquid crystal device havingthe above-described surface-emitting device 20 as a front light. Theliquid crystal device has a configuration in which the light guide plate22 of the surface-emitting device 20 is placed on the front side of areflective liquid crystal display panel 30 with a reflecting layer 31.When it is light outside, the surface-emitting device 20 transmits andguides external light into the reflective liquid crystal display panel30 so that a display produced in a display region V of the reflectiveliquid crystal display panel 30 can be viewed by the light reflected bythe reflecting layer 31. In contrast, when it is dark outside, sinceillumination light 22A can be applied from the lower side of the lightguide plate 22 toward the reflective liquid crystal display panel 30 bylighting the point light sources 21, a display produced in thereflective liquid crystal display panel 30 can be viewed by theillumination light 22A.

FIG. 3 is a partial sectional plan view of the above-describedsurface-emitting device 20. Three point light sources 21 are arranged atnearly regular intervals along one end face 22 c of the light guideplate 22. Each of the point light sources 21 has light emittingcharacteristics having directivity so that illuminance is highest in thefrontward direction, as shown by the arrows in the figure, and so thatilluminance rapidly decreases away from the frontward direction.Therefore, in a case in which illumination is performed using light,which travels from the light source 11 along the faces of the lightguide plate 12, as in the conventional front light shown in FIG. 5, whenthe point light sources 21 are adopted instead of the light source 11,sufficient in-plane uniformity of illumination light cannot be obtainedin a light-source-adjacent region Va of a display region V shown in FIG.4 close to the point light sources 21 of the light guide plate 22. Inthis case, a method is possible which yields sufficient in-planeuniformity of illumination light, even in the light-source-adjacentregion Va, by diffusing light by a light-scattering plate or the likeinterposed between the point light sources 21 and the light guide plate22. In a case in which the light-scattering plate is placed on theincident surface of the light guide plate, when a sufficient opticalpath length is ensured by placing the point light sources 21 at asufficient distance from the light guide plate, light scattered by thesurface of the light-scattering plate becomes equivalent to light from alinear light source, and therefore, in-plane uniformity can be obtained.In this case, while the light-scattering plate may provide a lowscattering intensity, this causes a structural problem because asufficient distance is required between the light guide plate and thepoint light sources. Since a light-scattering plate with a highscattering intensity is necessary to obtain similar advantages in astate in which the point light sources 21 are disposed adjacent to thelight guide plate, light loss is great, and it is difficult to obtainsufficient brightness. Conversely, an excessively large amount of poweris consumed in order to obtain sufficient brightness.

In this embodiment, when light is introduced from the point lightsources 21 into the light guide plate 22, as shown in FIG. 3, most ofthe light reaches an opposite end 22 d, is scattered and reflected bythe light-scattering plate 23 and the reflection plate 24 disposed atthe opposite end 22 d, and is radiated downward as illumination light22A from the steeply inclined faces 22 b. Since a long optical pathlength can be obtained from the point light sources 21 to thelight-scattering plate 23, even when the scattering intensity of thelight-scattering plate 23 is low, light scattered by thelight-scattering plate 23 becomes equivalent to light from the linearlight source. Accordingly, it is possible to obtain a sufficientin-plane uniformity of the illumination light 22A, and to reducenonuniformity of leakage light 22B, as shown in FIG. 7.

If directivity of the point light sources 21 is not very high, it ispossible to ensure a certain degree of in-plane uniformity ofillumination light and a certain degree of uniformity of leakage lightwithout using the light-scattering layer 23. The light-scattering effectmay be obtained by roughening the. surface (reflecting surface) of thereflection plate 24, or by making minute pits and projections on thesurface of the reflection plate 24 by causing another material toselectively adhere thereto, instead of using the light-scattering plate23. The end face 22 d of the light guide plate may be made into alight-scattering portion by forming minute pits and projections thereon.

In a case in which the density and shape of the light-emittingstructure, such as the above-described convex portions, gradually changealong the light traveling direction in order to make the in-planedistribution of illumination light uniform as the surface-emittingdevice, in this embodiment, they change along the traveling direction(that is, in the rightward direction in FIG. 1) of reflected light fromthe reflecting surface of the reflecting layer, such as the reflectionplate 24, whereby in-plane uniformity of the illumination light 22A canbe enhanced.

Industrial Applicability

As described above, in the surface-emitting device of the presentinvention, light led from the light source into the light guide platetravels inside the light guide plate, is reflected by the reflectionplate, travels nearly in a predetermined direction, and is emitted fromthe surface of the light guide plate. Since this can ensure a longoptical path length from the light source to the emitting position,in-plane uniformity of emitted light can be enhanced when the lightsource has directivity, when the light source is a point light source,or when the distribution of light guided from the light source into thelight guide plate is nonuniform. When this surface-emitting device isused as a front light placed on the front side of various displays,electrical engineering devices, and electronic devices, sincenonuniformity of the distribution of leakage light leaking toward thefront side is reduced, visibility can be improved.

What is claimed is:
 1. A surface-emitting device, comprising: atransmissive light guide plate that emits from its surface lightpropagating therein in a first direction along its plate plane, thefront surface of said light guide plate comprising a convex portion or aconcave portion having an inclined face that emits light from the frontsurface; a light source placed adjacent to a first end of said lightguide plate that introduces light into said light guide plate in asecond direction different from the first direction; and a reflectinglayer, disposed at a second end of said light guide plate different fromthe first end where said light source is placed, that reflects lightwhich propagates inside said light guide plate in the second directionin nearly the first direction, inside said light guide plate, saidconvex portion or said concave portion of said front surface of saidlight guide plate comprising a gently inclined face formed to face thesecond direction, as viewed from the inside of said light guide plate,and inclined at a small angle to the rear face of said light guideplate, and a steeply inclined face formed to face the first direction,as viewed from the inside of said light guide plate, and inclined at alarge angle to the rear face of said light guide plate.
 2. Thesurface-emitting device according to claim 1, further comprising alight-scattering layer interposed between said light guide plate andsaid reflecting layer.
 3. The surface-emitting device according to claim1, said light source having directivity in a light emitting directioninside said light guide plate.
 4. The surface-emitting device accordingto claim 1, said light source being a point light source.
 5. Thesurface-emitting device according to claim 3, said light source being alight-emitting diode.
 6. The surface-emitting device according to claim1, said light guide plate not emitting light traveling in the seconddirection from its surface before the light reaches said reflectinglayer.
 7. A front light, comprising: a transmissive light guide platethat emits from front surface light propagating therein in a firstdirection along its plate plane, the front surface of said light guideplate comprising a convex portion or a concave portion having aninclined face that emits light from the front surface; a light sourceplaced adjacent to a first end of said light guide plate that introduceslight into said light guide plate in a second direction different fromthe first direction, said light guide plate allowing sight therethrough;and a reflecting layer disposed at a second end of said light guideplate different from said first end where said light source is placed,that reflects light which propagates inside said light guide plate inthe second direction in nearly the first direction, inside said lightguide plate, said convex portion or said concave portion of said frontsurface of said light guide plate comprising a gently inclined faceformed to face the second direction, as viewed from the inside of saidlight guide plate, and inclined at a small angle to the rear face ofsaid light guide plate, and a steeply inclined face formed to face thefirst direction, as viewed from the inside of said light guide plate,and inclined at a large angle to the rear face of said light guideplate.
 8. The front light according to claim 7, further comprising alight-scattering layer placed before a reflecting surface of saidreflecting layer.
 9. The front light according to claim 7, said lightsource having directivity in a light emitting direction in said lightguide plate.
 10. The front light according to claim 7, said light sourcebeing a point light source.
 11. The front light according to claim 9,said light source being a light-emitting diode.
 12. The front lightaccording to claim 7, said light guide plate not emitting lighttraveling in the second direction from the surface before the lightreaches said reflecting layer.
 13. A liquid crystal device comprising: aliquid crystal pane: and the front light according to claim 7 providedin front of the liquid crystal panel.